There are a number of conventional processes for packaging surface mounted integrated circuits. One approach is commonly referred to as “flip chip” packaging which generally contemplates the forming of solder bump contacts (or other suitable contacts) directly on an integrated circuit die on a wafer. After the contacts are formed, the dice are singulated by sawing or cutting the wafer along the scribe lines. The individual die are then “flipped” and surface mounted or attached to a substrate, such as a printed circuit board. That is, the solder bumps on the die are aligned and mounted onto matching contacts on the substrate. The solder bumps are then reflowed to electro-mechanically connect the die to the substrate.
When a flip chip die is mounted to the substrate, an air gap typically remains between the die and substrate. This gap is commonly filled with material that is flowed into the gap in liquid form and is then solidified. This material is generally a mixture of a epoxy resin and small silica spheres and is often called underfill. A dispenser or nozzle is typically used to dispense the liquid underfill material at one edge of the die. The material then flows into the narrow gap due to capillary action and spreads across the flip chip die until finally the entire area of the gap between the die and substrate is filled. The underfill material is then cured. For more details on the aforementioned type of package, see for example U.S. Pat. No. 6,245,595, and co-pending U.S. application Ser. No. 09/359,214 filed Jul. 22, 1999 and Ser. No. 10/080,913 filed Feb. 21, 2002, all assigned to the same assignee of the present application and incorporated by reference herein for all purposes.
There are problems associated with aforementioned type of underfill process. For example, the operation of applying underfill must be repeated for each flip chip mounted onto a substrate. Repeating such an operation many times during manufacturing significantly increases costs. Also, as the underfill material flows past solder bumps to fill the gap, separation of silica spheres from resin may occur. The occurs because some silica spheres may become trapped as they meet solder ball obstacles. The underfill material may therefore develop streaks of high and low silica concentration. The silica may also separate from the resin by sinking to one side of the gap, thus creating a silica rich side in the bottom and a resin rich side on the top of the gap. This segregation of silica and resin alters the mechanical properties of the filled region and thereby may mitigate the mechanical function of the underfill. The underfill material is also a generally poor heat conductor. The use of flip chips are therefore typically limited to relatively low power, slow speed applications. Also the mis-match of the coefficients of thermal expansion between the silicon, underfill, and substrate tends to create stresses on the solder that could lead to joint failure. Finally, after a flip chip has been surface mounted to a substrate, the underfill material makes part replacement or reworking very difficult.
An apparatus and method is therefore needed for forming a wafer level solder bump around the periphery of each die on a flip chip wafer so that when an individual die is singulated from the wafer and mounted onto a substrate, the solder bump acts to hermetically attach and seal the flip chip to the substrate.